In an induction motor control circuit or the like for performing a DC.fwdarw.AC conversion by utilizing the switching of a transistor in a switching regulator, transistor inverter or the like, voltage undergoes a sudden change owing to the switching of the transistor and a high-frequency current is generated by the integrated component (dv/dt) of the voltage. This high-frequency current flows through the stray capacitance of the loading device (namely the capacitance between the device and earth) and through the capacitance between a power supply cable and earth, so that the power supply cable acts as an antenna and emits radio waves. These radio waves have an influence upon the carrier waves (500 KHz-2 MHz) of an AM radio and are a source of such noise as radio noise. It is desired that radio wave trouble caused by such transistor switching be prevented.
If an insulating transformer serving as means for countering radio wave trouble is provided on the side of the loading device, the emission of radio waves from the power supply cable can be prevented since the high-frequency current is not transmitted to the exterior of the loading device. However, an insulating transformer is disadvantageous in that it is costly, complicates the construction of the apparatus and results in an apparatus of larger size.
There has recently been proposed a noise absorbing circuit in which a low-pass filter comprising a capacitor and an AC reactor (inductance component) is arranged on the input side to cut the high-frequency current from a load circuit inside the apparatus so that this current is not transmitted to the power supply cable side.
According to such an arrangement, cutting of the high-frequency current caused by transistor switching can be achieved without using a transformer. Installation of a leakage current breaker is mandatory from the viewpoint of safety and prevention of fire caused by leakage current. The leakage current breaker is actuated when current exceeds 30 mA.
Consequently, when the low-pass filter is provided, there is the danger that a leakage current at the power supply frequency will be produced, thereby actuating the leakage current breaker. Further, though there is little risk of erroneous operation in a case where one apparatus is connected to one leakage current breaker, ordinarily a plurality of apparatus are connected to a single leakage current breaker. In such case, a problem arises wherein the leakage current breaker malfunctions due to a low-pass filter leakage current having a power supply frequency component. Thus, the state of the art is such that technical difficulties are encountered in fabricating a satisfactory high-frequency noise absorbing circuit.